[17.05] Weather on Titan

Titan's atmosphere potentially sports a cycle similar to the
hydrologic one on Earth with clouds, rain and seas, but with
methane playing the terrestrial role of water. Over the past
ten years many independent efforts indicated no strong
evidence for cloudiness until some unique spectra were
analyzed in 1998 (Griffith et al.). These surprising
observations displayed enhanced fluxes of 14-200% on two
nights at precisely the wavelengths (windows) that sense
Titan's lower altitude where clouds might reside. The
morphology of these enhancements in all 4 windows observed
indicate that clouds covered ~6-9% of Titan's surface
and existed at ~15 km altitude.

Here I discuss new observations recorded in 1999 aimed to
further characterize Titan's clouds. While we find no
evidence for a massive cloud system similar to the one
observed previously, 1%-4% fluctuations in flux occur
daily. These modulations, similar in wavelength and
morphology to the more pronounced ones observed earlier,
suggest the presence of clouds covering \leq1% of Titan's
disk. The variations are too small to have been detected by
most prior measurements. Repeated observations, spaced 30
minutes apart, indicate a temporal variability observable in
the time scale of a couple of hours. The cloud heights hint
that convection governs their evolutions. Their short lives
point to the presence of rain.

C. A. Griffith and J. L. Hall are supported by the NASA
Planetary Astronomy Program NAG5-6790.